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J. Phys. Chem. B 2002, 106, 6178-6183
A New Way to Prepare Nanostructured Materials: Flame Spraying of Microemulsions M. Bonini, U. Bardi, D. Berti, C. Neto, and P. Baglioni* Department of Chemistry and CSGI, UniVersity of Florence, Via della Lastruccia 3 - Sesto Fiorentino, 50019 Florence, Italy ReceiVed: June 1, 2001; In Final Form: January 8, 2002
In this report we describe a new method to obtain nanostructured coatings or powders based on the flame decomposition of microcompartmentalized solutions. Metal nanoclusters of well-defined size are obtained by reduction of a metal salt inside the water compartment of water in oil (w/o) microemulsions, formed by water in hexane and stabilized by an appropriate surfactant. Metal nanoclusters can be separated from the mother solution by spraying the microemulsion solution into an air/acetylene flame. In this way, nanostructured coating or powder, almost preserving the original structure of the nanoparticles synthesized in the microemulsion system, can be obtained in quantities sufficient for industrial applications. As an example we report the flame spraying of gold microemulsions to produce gold coating onto silicon wafers. To the best of our knowledge, this study reports a new method allowing the use of a microemulsion synthetic pathway for the production of consistent amount of nanoparticles. This method could be of great utility in many applications involving nanoparticles in the fields of physics, chemistry, biotechnology, and biology.
Introduction Nanomaterials have a relevant importance for industrial applications since they have unique mechanical, optic, and magnetic properties. It is well-known that certain physical and mechanical properties of materials considerably change as their grain size is reduced. For example, improvements in hardness, ductility, and dielectric properties can be obtained. Unfortunately, at present only a few methodologies are available for the synthesis of nanostructured powders or coatings at the industrial level.1 Nanopowders can be directly synthesized by using a microemulsion system.2-5 This method is probably the most powerful because it allows the size and shape of the formed nanoparticles to be controlled. However, the separation of the nanoparticles from the mother microemulsion solution has proven to be very difficult and, even after a tedious procedure of separation, the quantity of powder obtained is scarce and cannot be used for industrial purposes. Another method to obtain nanostructured coatings or powders consists in the use of thermal spraying systems.6 Thermal spraying is a complex process that combines particle injection, melting, quenching, and consolidation in a single process. Thermally heated and melted particles are propelled toward a substrate (or simply obtained as a powder) where they are flattened and quenched in a very short time. The gases, combustion gases, or plasma are the sources of the thermal and kinetic energy heating the particles and propelling them to the substrate. As in any coating process, the properties of the coating are a complex function of the process variables. Key parameters are the spray characteristics, the chemical composition, the surface preparation and the temperature of the substrate, the * Author to whom correspondence should be addressed. E-mail:
[email protected].
physical properties of the feeding materials (especially density), melting point, and latent heat of fusion. Moreover, the flame temperature and the plasma electric power are fundamental in controlling the final product quality. One of the most common thermal spraying methods operates with powders7-9 as feeding material in a plasma or flame spraying system. With this method the operating conditions can be imposed so that the individual grains do not melt upon impacting the substrate. The main drawback of this procedure is that it requires powders with grain sizes greater than 1-5 µm, because of the difficulty in handling powders of smaller grain size. Conventional plasma spraying processes normally use a 10-100 µm sized feedstock to form the coating. When finer particles are used, they have a tendency to flow badly or to agglomerate heavily. The use of liquid solutions instead of powders can in principle overcome this problem.10 However, the “liquid method” has obtained limited industrial success because of the poor control of the structure of the coated film obtainable by the conventional technology. In particular, the only way to control the grain size of films obtained by flame spraying of a liquid is to control the droplet size, which is really a demanding task. Furthermore, when the droplets are into the flame, they may undergo complex phenomena that are also very difficult to control. Direct continuous ink jet printers have been used to obtain uniformly spaced droplets by superimposing a periodic disturbance on a high velocity ink stream.11 Also dispersion of powders in various solvents has been used as feedstock materials,11 but in any case it has not been possible to obtain materials containing nanostructured “units” lower than about 5 µm. In this report we propose an innovative method, consisting in the firing of a microemulsion. This allows production of large quantities of 10-200 nm monodisperse nanoparticles with a good control of the coated films. This new method can be used
10.1021/jp012098p CCC: $22.00 © 2002 American Chemical Society Published on Web 05/25/2002
Preparation of Nanostructured Materials to produce both nanopowders and nanostructured coatings. As an example of the new method to prepare nanostructured gold coatings by flame spraying, we describe two different kinds of microemulsions, both formed by water in hexane. A gold salt is directly reduced into reverse micelles of ammonium laurate or Brij30 by using hydrazine hydroxide. UV-Vis spectroscopy, light scattering, and transmission electron microscopy (TEM) investigations show the formation in the microemulsions of metallic nanoclusters having diameters between 10 and 100 nm. Suspensions so prepared are then used to feed the flame spraying system and to create nanostructured gold films upon a silicon substrate. Gold films have been investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Results clearly demonstrate that the size of the nanoclusters, present in the microemulsion systems, is preserved upon microemulsions flame spraying, allowing the preparation of nanomaterials of controlled size and shape starting from a relatively simple methodology. In our opinion, this new process for production of nanosized materials overcomes the intrinsic limitations of most of the methods currently used for nanoparticles and nanocoating production. This new method will extend the application of nanotechnologies from the laboratory to industrial scale in chemistry, biology, biotechnologies, and physics. Experimental Section The reagents have been chosen in order to obtain a microemulsion that can be easily fired. Brij30, a nonionic surfactant with a short hydrocarbon chain, (tetraethyleneglycododecyl ether: CH3(CH2)11(CH2CH2O)4OH purchased from Fluka, purity >95%), and ammonium laurate, anionic surfactant with a relatively short chain and without a metal counterion, (C11H23COO- NH4+), were employed to stabilize microemulsions. Ammonium laurate (Pfalts & Bauer, Inc.) was recrystallized three times from ethanol before use. Hydrazine hydroxide (Merck-Schuchardt, purity >99%) was employed as reducing agent since it can be completely decomposed once fired. The continuous oil phase was hexane (Sigma-Aldrich, purity 95% HPLC grade). Tetrachloroauric acid was purchased from Aldrich Chem. Co. (purity >99%). Gold nanoparticles have been prepared using the well-known microemulsion technology.2,3,12,13 The synthesis of the nanoclusters consists of mixing two w/o microemulsions with the same w0 (w0 ) [H2O]/[surfactant]) value, one containing tethrachloroauric acid and the other containing hydrazine hydroxide. Microemulsions containing the metallic salt have been prepared by adding different amounts of a 1 M aqueous solution of HAuCl4‚3H2O to a 0.2 M surfactant solution in hexane, to obtain the desired w0 value. The final metal concentration, expressed as moles of metallic salt over total volume, was always of the order of 10-3 M. In the same way, microemulsions containing the reducing agent have been prepared by adding an aqueous solution of hydrazine hydroxide to a 0.2 M surfactant solution in hexane. The concentration of hydrazine was 10 times higher than that of gold because a lower stoichiometric ratio caused the slow precipitation of gold clusters. Mixing the two microemulsions produces a colloidal suspension of gold nanoclusters. The reaction is instantaneous in the case of Brij30, while it is much slower with ammonium laurate. In both cases, once gold reduction is complete, the colloidal suspension is deep red colored, as typically observed for suspensions of gold nanoclusters.
J. Phys. Chem. B, Vol. 106, No. 24, 2002 6179 The temperature has been kept at 25 °C during the entire preparation for the microemulsion with Brij30 and at 30-35 °C for ammonium laurate, since in the latter case microemulsions are not stable at lower temperatures, being the reduction of gold salt too slow. The formation of gold nanoparticles has been confirmed by UV-Vis spectra obtained with a Lambda 5 Perkin-Elmer Spectrometer. An Applied Photophysics SX 18MV Microvolume Stopped-Flow Reaction Analyzer was used to follow kinetic processes for time scales
